1. Field of the Invention
The present invention is directed towards novel sulfonic acid esters as catalysts for crosslinking of polymers having hydroxyl, carboxyl, and amide functional groups with a crosslinking agent and resin compositions containing the same. More particularly, the present invention is directed to the specific use of these catalysts in the crosslinking or curing of hydroxyl, carboxyl or amide containing polymers with an acid catalyzed amino-formaldehyde crosslinking agent, such as, hexamethoxymethylmelamine (“HMMM”), producing polymeric film coatings that have superior hardness, impact resistance, adhesion, improved blister resistance, salt spray characteristics and flexibility.
2. Background Information
Coating compositions, which are capable of acid catalyzed cross-linking generally contain an effective catalytic amount of an acid catalyst. The acid catalyst is added in order to accelerate crosslinking and, thereby, reduce the overall time required for cure. When an acid catalyst is utilized, it is advantageous to have the catalyst present as the free acid because in this manner a rapid cure can be efficiently achieved. However, the presence of the free acid may also cause problems in storage stability, that is, the coating composition will exhibit a tendency to gel and harden during the storage term, thus becoming unfit for use.
Robert Saxon et al., J. of App. Polymer Sc., 8, pp. 475-488 (1964), reported the use of p-toluenesulfonic acid (p-TSA) as a curing catalyst for HMMM alone or in combination with acrylic polymers. It was found that although curing can take place without the presence of an acid catalyst, the rate of cure is very slow and the curing temperature required is very high. The presence of p-TSA lowered the curing temperature and increased the cure rate. However, resin compositions containing p-TSA are unstable at room temperature and require mixing of the ingredients immediately prior to use. This makes it difficult to control the quality of the resin produced and to put together a stable one package coating system.
As a means of circumventing these difficulties, latent or blocked acid catalysts are often utilized to delay the action of crosslinking agents and otherwise postpone the curing mechanism. Latent acid catalysts are formed by preparing a derivative of an acid catalyst, such as p-toluenesulfonic acid (pTSA) or other sulfonic acids. For example, a well-known group of blocked acid catalysts are amine salts of aromatic sulfonic acids. Such sulfonate salts are less active than the free acid in promoting crosslinking. During cure, the catalysts are activated by heating, which results in liberation of the free sulfonic acid catalyst, and the volatile blocking group.
U.S. Pat. No. 4,200,729, and U.S. Pat. No. 4,251,665, for example, describe curing amino resins, e.g., urea-formaldehyde and melamine-formaldehyde thermosetting resin systems, with thermally-decomposable adducts of aromatic sulfonic acids and oxa-azacyclopentanes. The resin products cured with these adducts provide surface coatings having superior water resistance properties.
Grant et al., U.S. Pat. No. 3,842,021 and Parsons et al., U.S. Pat. No. 3,868,338 describe epoxy blocked p-TSA as a catalyst for the curing of polyester-HMMA resins. The catalyst is prepared from p-TSA wherein the acid group is blocked with a compound containing an oxirane group, specifically a bisphenol-A epichlorhydrin epoxy resin. Sulfonic acid catalysts blocked with other groups such as 2-hydroxy-cycloalkyl or aryl substituted 2-hydroxy cycloalkyl groups (U.S. Pat. No. 4,469,832); alpha-hydroxy carbonyl groups (U.S. Pat. No. 4,510,290); beta-hydroxy free groups (U.S. Pat. Nos. 4,454,274 and 4,477,618) and beta-hydroxy carbonyl groups (U.S. Pat. No. 4,504,372) also have been reported.
The compositions of U.S. Pat. Nos. 4,192,826 and 4,281,075 are exemplary of coating compositions that utilize a latent acid catalyst to delay the action of the crosslinking agent until cure. U.S. Pat. No. 4,192,826 is directed to thermosetting liquid coating compositions based on polymer capable of being crosslinked with nitrogen resin crosslinkers, and blocked acid catalyst prepared from materials containing at least one oxirane functionality and a sulfonic acid. The blocked acid catalyst resulting from these reactants is a beta-hydroxy sulfonic acid ester. U.S. Pat. No. 4,281,075 is directed to thermosetting compositions based on a film-forming component bearing or capable of generating hydroxyl functionality, amine-aldehyde crosslinking agent, and a latent acid catalyst having the formula:A—(SO2)—O—Xwherein A is selected from alkyl or aryl groups and X is selected from 2-hydroxycycloalkyl and aryl substituted 2-hydroxycycloalkyl groups. The latent acid catalysts depicted by this formula are also beta-hydroxy sulfonic acid esters.
Also exemplary of coating compositions utilizing latent acid catalysts are GB No. 1,361,929 and GB No. 1,413,054.
U.S. Pat. No. 4,469,832 describes a high solids, organic solvent based, sprayable coating composition, which is capable of acid catalyzed crosslinking and is composed of an active hydrogen-containing resin, a curing agent present externally and/or internally as a part of the active hydrogen-containing resin, and a catalytic amount of a non-ionic, saturated or unsaturated organic polycyclic ester of a sulfonic acid.
U.S. Pat. No. 4,454,274 describes a coating composition that is capable of acid catalyzed crosslinking, comprising an active hydrogen-containing resin, a curing agent present externally and/or internally as a part of the active hydrogen-containing resin, and a catalytic amount of a non-ionic cycloaliphatic sulfonic acid ester as the latent acid catalyst.
U.S. Pat. No. 4,501,854 describes a curable composition capable of acid catalyzed crosslinking, comprising an active hydrogen-containing resin, a curing agent present externally and/or internally as a part of the active hydrogen-containing resin, and an acid catalyst, wherein the improvement is alleged to comprise using a catalytic amount of disulfonic acid esters as the latent acid catalyst.
U.S. Pat. No. 4,477,618 describes an organic solvent based coating composition having a total solids content of at least 50 percent and which is sprayable, the coating composition being capable of acid catalyzed crosslinking, comprising an active hydrogen-containing resin, a curing agent present externally and/or internally as a part of the active hydrogen-containing resin, and a catalytic amount of a non-ionic ester of a sulfonic acid.
U.S. Pat. No. 4,812,215 describes an aqueous, cationically electrodepositable coating composition, comprising a ternary sulfonium or quaternary phosphonium group containing resin, an aminoplast or phenoplast crosslinking agent and a catalytic amount of a non-ionic ester of a sulfonic acid as latent acid catalyst.
However, there stills remain a need within the art for coating compositions capable of being acid catalyzed cross-linked having improved storage stability and reduced time required for cure.